Add test case for nested structs

[golanglemonade]

Adds a test for #219

On version v0.21.2:

=== RUN   TestMarshalJSON/marshal_nested
runtime: goroutine stack exceeds 1000000000-byte limit
runtime: sp=0x140201e0380 stack=[0x140201e0000, 0x140401e0000]
fatal error: stack overflow

On latest:

    --- PASS: TestMarshalJSON/marshal_NumberOne (0.00s)
    --- PASS: TestMarshalJSON/marshal_NumberTwo (0.00s)
    --- PASS: TestMarshalJSON/marshal_nested (0.00s)
    --- PASS: TestMarshalJSON/marshal_a_struct_with_custom_marshaler (0.00s)
    --- PASS: TestMarshalJSON/marshal_map_with_custom_marshaler (0.00s)
    --- PASS: TestMarshalJSON/marshal_time.Time (0.00s)
    --- PASS: TestMarshalJSON/marshal_time.Time#01 (0.00s)
PASS
ok  	github.com/Yamashou/gqlgenc/clientv2	0.140s